Method for prestressing tubes of a heat exchanger with precise tailoring of the prestress

ABSTRACT

In order to prestress a tube ( 4 ) of a heat exchanger in tension in the axial direction, with precise tailoring of the prestress, at least one of the end parts ( 4   a,    4   b ) of the tube ( 4 ) is expanded diametrally using a hydraulic pressure while the tube ( 4 ) is fixed at its ends in the tube plates ( 3   a,    3   b ). The value of the prestress in each of the tubes ( 4 ) of the heat exchanger is tailored by altering the initial radial clearance between the external surface of the end part ( 4   a,    4   b ) of the tube ( 4 ) and the internal surface of the hole ( 5 ) passing through the tube plate ( 3   a,    3   b ) in which hole the end part ( 4   a,    4   b ) of the tube ( 4 ) is engaged before diametrally expanding the end part. The end part ( 4   a,    4   b ) of the tube which runs substantially over the entire length of the hole ( 5 ) passing through the tube plate ( 3   a,    3   b ) is hydraulically diametrally expanded. The initial radial clearance ( 6 ) between the tube ( 4 ) and the hole ( 5 ) is tailored to obtain longitudinal contraction of the tube ( 4 ) and therefore a prestress of a precisely determined value. The invention applies in particular to once-through heat exchangers.

[0001] The invention relates to a method for prestressing tubes of aheat exchanger in tension in a longitudinal direction, with precisetailoring of the prestress in the tubes.

[0002] Heat exchangers used in particular as steam generators in nuclearreactors and which comprise a great many tubes which are fixed at theirends into at least one tube plate of the heat exchanger are known.

[0003] Such steam generators may be very large, for example in excess of20 metres tall with a diameter barely smaller than 4 metres.

[0004] Such heat exchangers may, in particular, be heat exchangers withstraight tubes (known as “once-through” exchangers), each of the tubesof the bundle being fixed, at a first end, into a first tube platearranged in the upper part of the steam generator in the serviceposition and, at a second end, into a second tube plate arranged at thelower part of the steam generator in its service position.

[0005] In such once-through generators, the tube bundle may contain overfifteen thousand tubes of a length in excess of 15 metres. The ends ofthe tubes are fixed in holes passing through the tube plates over theirentire thickness in the axial direction of the bundle and open at theirends into water boxes delimited by a generally hemispherical wall and bythe corresponding tube plate.

[0006] Steam generators of another type comprise a bundle of tubes bentinto the shape of a U which are fixed at both ends into two holes in thesame tube plate.

[0007] It is known for the ends of the tubes of the steam generators tobe fixed into the holes passing through the tube plates by crimping andwelding inside the tube plates.

[0008] It is also known for diametral expansion to be performed on theend parts of the tubes of heat exchangers as described above, inside theholes passing through the tube plate, by hydraulic means, the internalsurface of the tube being subjected to the pressure of a hydraulicfluid, generally water under very high pressure. This diametralexpansion operation, which is generally carried out on an end part ofthe tube stretching along the entire length of the hole passing throughthe tube plate, may be used to crimp the tube into the hole or to pressthe wall of the tube perfectly firmly against the wall of the hole,particularly on the interior face side of the tube plate, so as toprevent there being any gap between the tube and the tube plate as thisgap could prove responsible for crevice corrosion while the steamgenerator is running, the external surface of the tubes coming intocontact with one of the heat-exchange fluids the corrosive capability ofwhich may be high at gaps between the tube and the tube plate.

[0009] In any case, diametral expansion of a tube produces contractionof the tube in the axial or longitudinal direction which means that ifthe tube is fixed at both ends into tube plates, the separation of whichis fixed, the tube is prestressed in longitudinal tension as a result ofthe hydraulic diametral expansion.

[0010] In heat exchangers such as once-through exchangers, it isgenerally desirable to dictate a certain longitudinal tensile prestressfor the tubes, so as to limit the buckling in service as a result of thedifferential expansions of the heat exchanger wrapper and/or of the tubeplates and of the tubes and the vibrations produced by the circulationof the heat-exchange fluids in contact with the walls of the tubes.

[0011] The tubes of once-through exchangers are also held by transversespacer plates distributed along the longitudinal direction of the heatexchanger. The buckling of the tubes may therefore give rise to damageto the tubes by bringing these into contact with the edges of theopenings in the spacers.

[0012] The longitudinal tensile prestress of the tubes needs to betailored to a precise value determined so as to limit the buckling andvibrations and also possibly to limit and balance the axial forcesexerted by the tubes on the tube plates of the once-through heatexchanger.

[0013] To tailor the value of the tensile prestress introduced into thetubes by the hydraulic diametral expansion, U.S. Pat. No. 6,357,114proposes altering the length of the end part of the tube to whichhydraulic expansion is applied so as to obtain the desired contractionand therefore the desired axial prestress in the tube subjected to thediametral expansion. In carrying out this method, the depth to which theexpansion tool is introduced into at least one of the end parts of thetube from the external face of the tube plate is tailored. The diametralexpansion is generally performed at both ends of the tube, already fixedto the tube plates, which means that the two hydraulic expansionoperations work together in prestressing the tube. The prestress can betailored by altering the length of hydraulic expansion of just one ofthe two ends of the tubes in one of the tube plates.

[0014] In all cases, there remains, at one of the ends of the tube, apart of the wall of the tube which is not pressed firmly against theinternal surface of the hole in the tube plate, and this manifestsitself in the presence of a gap which generates crevice corrosion whilethe steam generator is running.

[0015] Furthermore, carrying out the hydraulic expansion operation iscomplex, because it is necessary to tailor the depth to which thediametral expansion tool is introduced into at least one of the ends ofthe very numerous tubes of the heat exchanger before diametrallyexpanding them.

[0016] The object of the invention is therefore to propose a method forprestressing, in longitudinal tension, tubes of a heat exchangercomprising at least one straight part and fixed at their ends in holespassing through at least one tube plate, in which the tensile prestressis obtained by hydraulically diametrally expanding at least one end partof each of the tubes of the heat exchanger fixed at its two ends in theat least one tube plate in such a way as to induce deformation throughlongitudinal contraction of the tube by subjecting the internal surfaceof the tube to a hydraulic pressure and in such a way that the prestressof the tube is tailored to a precise value, this method making itpossible to avoid any risk of crevice corrosion of the tubes in the heatexchanger in service and to make the operation of effecting tailoredprestressing of the tubes easier.

[0017] To this end, the value of the prestress in each of the tubes ofthe heat exchanger is tailored by altering the initial radial clearancebetween the external surface of the end part of the tube and theinternal surface of the hole passing through the tube plate in whichhole the end part of the tube is engaged before diametrally expandingthe end part and the tube is hydraulically diametrally expanded alongthe entire end part of the tube which extends substantially over theentire length of the hole passing through the tube plate.

[0018] The invention also relates, in general, to a method forprestressing, in longitudinal tension, tubes of small diameter and longlength comprising at least one straight part, so as to increase thebuckling margin and reduce induced vibrations, and which are fixed attheir ends in holes passing through at least one tube plate, in whichmethod the tensile prestress is obtained by hydraulically diametrallyexpanding the ends of each of the tubes so as to induce deformationthrough longitudinal contraction distributed uniformly along the lengthof the tube by subjecting the internal surface of the tube to ahydraulic pressure and in such a way that the prestress of the tube istailored to a precise value by predetermined diametral expansion of theend part and that the tube is hydraulically diametrally expanded.

[0019] In order to make the invention easy to understand, severalembodiments of the method according to the invention for prestressingtubes of a once-through steam generator of a nuclear reactor will bedescribed by way of example.

[0020]FIG. 1 is a view in vertical axial section of a once-through steamgenerator of a nuclear reactor.

[0021]FIG. 2 is a plan view of one of the tube plates of the heatexchanger.

[0022]FIGS. 3 and 4 are details on a larger scale of part of the tubeplate depicted in FIG. 2.

[0023]FIG. 3 is a part view of the internal face of the tube plate,prior to diametral expansion of the tubes of the steam generator.

[0024]FIG. 4 is a part view of the internal face of the tube plate afterdiametral expansion of the end parts of the tubes of the steamgenerator.

[0025]FIGS. 5A, 5B, 5C, 5D and 5E are part views in axial section,during five successive steps of a method for fixing and prestressing atube of the once-through steam generator, according to the invention andaccording to a first embodiment.

[0026]FIGS. 6A, 6B and 6C are views in axial part section of a tube ofthe steam generator during three successive steps of a method for fixingand prestressing the tube, according to the invention and according to asecond embodiment.

[0027]FIG. 1 schematically depicts a once-through steam generator usingthe overall reference 1, which steam generator comprises a wrapper ofcylindrical overall shape 2 connected at a first end or upper end to anupper tube plate 3 a and at a second end or lower end to a lower tubeplate 3 b.

[0028] The tube plates 3 a and 3 b are pierced with a great manythrough-openings 5 arranged in a triangular-pattern network as can beseen in particular in FIGS. 2, 3 and 4. The openings 5 could just aseasily be arranged in a square-pattern network. The straight tubes 4 ofthe steam generator bundle are fixed, by a first end part, inside theupper plate 3 a and, by a second end part, in a through-opening in thelower plate 3 b.

[0029] The plates 3 a and 3 b have an internal face facing toward theinside of the steam generator containing the tube bundle 4 and anopposite external face constituting a wall delimiting a water box of thesteam generator for distributing or collecting a heat-exchange fluid.

[0030]FIG. 2 depicts the internal face of a tube plate of circularoverall shape 3 a (or 3 b) which has a great many through-holes (forexample over fifteen thousand holes). The holes 5 are distributeduniformly between a central zone and a peripheral zone of the tube plate3 a (or 3 b). The holes 5 passing through the tube plate through itsentire thickness and arranged in the triangular-pattern network arevisible in particular in FIG. 3 which shows the end parts of tubes 4engaged in the openings 5 with a certain radial (or diametral) clearance6.

[0031]FIG. 3 depicts the internal face of a tube plate in which the endsof the tubes are engaged, in a stage of the mounting of the bundle ortubing prior to the carrying-out of the diametral expansion step in themethod according to the invention.

[0032]FIG. 4 depicts a part view, similar to the view of FIG. 3, whenthe diametral expansion operation according to the invention has beencarried out, the clearance 6 having therefore been eliminated and thetubes being pressed firmly against the interior surface of thecorresponding holes 5.

[0033] An operation of mounting a tube of a steam generator, duringwhich the tube is prestressed according to the invention and accordingto a first embodiment will now be described with reference to FIGS. 5Ato 5E.

[0034] In a first step of the tubing method, the tubes are introducedinto the tube plates and the wrapper of the steam generator which havebeen preassembled. At the end of this first step of the method, asdepicted in FIG. 5A, the respective end parts 4 a and 4 b of the tubes 4are introduced into the openings 5 of the respective tube plates 3 a and3 b. It must be noted that the mounting clearance 6, between theexternal surface of the end part of the tube 4 and the surface of thehole 5 passing through the tube plate 3 a (or 3 b) allows the tube to beintroduced with ease through the openings in the tube plates, withoutdamaging the exterior surface of the tube which, at the end of theoperation, needs to have a perfect surface finish.

[0035] According to the invention, the size of the diametral (or radial)clearance 6 is tailored very precisely so as to achieve prestressing ofthe tube with a prestress of an extremely precise predetermined amount;this prestress results from a longitudinal contraction distributeduniformly over the length of the tube.

[0036] For that, first of all, the radial clearance needed in order toobtain the desired prestress value in the tube 4 is determined. Thisvalue of the radial clearance is obtained by calculations and by testson a model comprising test tube fixing components similar to tubeplates.

[0037] It has been possible to establish a very good correlation betweenthe results obtained by calculation and the results of measurementsobtained from the model.

[0038] The desirable actual radial clearance 6 between the tube and thethrough-holes 5 in the tube plates in which it is housed is obtained bymatching the diameter with which the hole 5 is pierced to the outsidediameter of the tube, taking account of the manufacturing tolerances onthe tubes and reserving the desired clearance 6.

[0039] As will be explained later on, the mounting clearances 6 for thevarious tubes may be designed to obtain an identical prestress in eachof the tubes of the bundle, regardless of the position of the tube or,on the other hand, to alter the value of the prestress according to theposition of the tube in the bundle or according to other particularconditions involved in the mounting of the tubes.

[0040] As depicted in FIG. 5B, having fitted the tube inside the steamgenerator, the end parts 4 a and 4 b of the tube being inside theopenings 5 in the corresponding tube plates 3 a and 3 b, diametralexpansion is carried out inside the opening 5 of a tube plate (forexample plate 3 a) on an end 4′a of the tube in a zone adjacent to theexternal face of the tube plate 3 a. Diametral expansion of the end 4′aof the tube 4 may be carried out by expansion rolling, for example usinga roller-type expander. At the end of the expansion-rolling operation,the fixing end 4′a of the tube 4 is pressed firmly against an externallayer 3 a′ of the plate 3 a made of a material which is chemicallycompatible with the material of the tube 4 a which is, for example, anickel alloy. After the expansion rolling of the end 4′a of the tube 4,a circular weld 7, for example using fillerless TIG welding, is used tofix the end of the tube 4 in a permanent and sealed fashion to theexternal face of the tube plate 3 a.

[0041] As can be seen from FIG. 5C, diametral expansion is thenperformed on the end part 4 a of the tube 4 which extends over theentire length of the hole 5 passing through the tube plate 3 a. Thisoperation is carried out by introducing a tool for exerting hydraulicpressure inside the end part of the tube 4, so that the internal surfaceof the tube 4 is subjected to the hydraulic pressure of a fluid, forexample water, carrying out its hydraulic diametral expansion along theentire axial length of the hole 5. At the end of the operation of adiametral expansion in the first tube plate 3 a, the end part 4 a of thetube 4 is perfectly firmly pressed against the surface of the hole 5,which means that no gap remains on the internal face of the tube plate 3a between the external surface of the tube and the opening 5 in the tubeplate 3 a.

[0042] During the diametral expansion operation on the end part 4 a ofthe tube, the contraction of the tube in its axial direction 8 ismanifested only in a slight displacement of the lower end 4 b of thetube which is free inside the opening 5 in the second tube plate 3 b. Noprestress is therefore introduced into the tube.

[0043] As can be seen in FIG. 5D, in a later step of the tubing, afixing end part 4′b of the tube 4 is expansion-rolled near the externalface of the second tube plate 3 b, then this end 4′b of the tube 4 iswelded with a circular weld 9 near the external face of the tube plate 3b.

[0044] As can be seen in FIG. 5E, the second end part 4 b of the tube 4is then diametrally expanded inside the opening 5 in the second tubeplate 3 b, over the entire axial length of the opening 5. The part 4 bof the tube is perfectly firmly pressed against the wall of the opening5 and no gap remains between the external surface of the tube and thehole 5, near the internal face of the tube plate 3 b.

[0045] During this second operation of hydraulic diametral expansionduring which use is made of a hydraulic expansion tool, as in the caseof the first end part 4 a of the tube inside the first tube plate 3 a,deformation is produced by contraction of the tube in the axialdirection, this deformation of the tube, which is fixed at its ends tothe tube plates 3 a and 3 b, producing a tensile prestress distributedalong the entire length of the wall of the tube 4 between the tubeplates 3 a and 3 b.

[0046] Furthermore, because of the initial clearance 6 between theexternal surface of the end part 4 b of the tube 4 and the opening 5 inthe tube plate 3 b has been tailored to obtain deformation bycontraction of a desired amount, the prestress in the tube is itself ata perfectly determined value which has been obtained by calculation andchecked by tests on models.

[0047] According to the invention, a prestress of a predeterminedprecise amount is therefore obtained by controlling the clearance andcarrying out radial expansion along the entire length of the holespassing through the tube plates, which means that no gaps that generatecrevice corrosion are present at the ends of the tube.

[0048]FIGS. 6A, 6B and 6C depict three successive steps in an operationof fitting a tube during the tubing of the steam generator during whichthe tube is prestressed according to the method of the invention andaccording to a second embodiment.

[0049]FIG. 6A is similar to FIG. 5A and will not be described again. Atthe end of the operation depicted in FIG. 6A, the tube 4 is in placeinside the steam generator, its end parts 4 a and 4 b being engaged inopenings 5 in the corresponding tube plates 3 a and 3 b.

[0050] The clearance 6 with which the ends 4 a and 4 b of the tube 4 aremounted inside the openings 5 of the tube plates 3 a and 3 b has beendetermined and set in such a way as, during subsequent diametralexpansion operations which will be described hereinbelow, to produce thedesired deformation of the tube in axial contraction and a desiredpredetermined level of prestress.

[0051] As depicted in FIG. 6B, in a second step of the tubing method,the ends 4′a and 4′b of the tube are expansion-rolled near the externalfaces of the tube plates 3 a and 3 b, so as to press the ends of thetube 4 firmly against the external layers of the tube plates. Thecircular connecting welds 7 and 9 connecting the respective ends 4′a and4′b of the tube 4 and the respective external faces of the plates 3 aand 3 b are then performed.

[0052] The tube 4 is therefore fixed at both ends to the plates 3 a and3 b, the separation of which is fixed because of the mounting of theplates on the structure of the steam generator.

[0053] As depicted in FIG. 6C, in a final step of the method, the twoend parts 4 a and 4 b of the tube 4 are hydraulically diametrallyexpanded in the corresponding openings 5 in the tube plates 3 a and 3 bover the entire axial length of the openings 5 so that, at the end ofthe hydraulic expansion operations, the parts 4 a and 4 b are perfectlyfirmly pressed over the entire length of the holes 5, no residual gapbeing present at the internal faces of the tube plates 3 a and 3 b.

[0054] During the operation of diametral expansion of the end part 4 aat the tube plate 3 a and of the end part 4 b at the tube plate 3 b, thetube 4, which is fixed at both ends, undergoes deformation incontraction in the axial direction 8, which means that the tube isprestressed during both of the diametral expansion operations.

[0055] The effects of the prestressing of the tube 4 during the firstdiametral expansion operation performed on the first end part 4 a of thetube 4 and the second diametral expansion operation performed on thesecond end part 4 b of the tube 4 combine so that the final prestress isthe result of the cumulative effect of the two diametral expansionoperations.

[0056] The initial clearance 6 with which the ends of the tube 4 aremounted in the openings in the tube plates is determined, by calculationand by tests on models, so that the total prestress obtained by the twodiametral expansions is precisely the desired prestress.

[0057] In the case of the first embodiment, only the second diametralexpansion operation generates the deformation by contraction of the tubeand the prestress, whereas in the case of the second embodiment, the twodiametral expansion operations combine to generate the prestress.

[0058] The method according to the invention therefore makes it possiblevery precisely to set the level of axial prestress on the tubes of aheat exchanger bundle while at the same time avoiding the presence ofgaps at the internal faces of the tube plate or plates.

[0059] Furthermore, the required prestress for the tubes or thedistribution of the prestresses on the tubes in the bundle can beobtained in a simple way by piercing holes in the tube plates with aperfectly defined and programmed diameter or with perfectly defined andprogrammed diameters, for example dependent on the position of the holesin the plane of the tube plates, bearing in mind the manufacturingtolerances on the sets of tubes used for tubing the heat exchanger.

[0060] The method according to the invention makes it possible inparticular easily to obtain perfectly constant prestress for all of thetubes of the heat exchanger.

[0061] The method according to the invention also makes it possible forthe prestresses of the tubes to vary, particularly according to theirposition in the bundle determined by the position of the openingsaccommodating the tube in the tube plates.

[0062] A suitable distribution of the prestresses of the tubes in thebundle makes it possible in particular to limit the deformation of thetube plates under the effect of the tensile forces exerted by the tubeson the tube plates. Constant prestress in all of the tubes of the bundlemay manifest itself in flexural deformation of the tube plates subjectedto high forces in their peripheral part. This effect can be correctedfor by adopting an appropriate distribution of prestresses in the tubesof the bundle. Likewise, in-service deformations of the various parts ofthe heat exchanger, for example under the effect of differentialexpansion, may be limited by adopting a suitable distribution of theprestresses in the tubes of the bundle.

[0063] Furthermore, to limit the deformations of the tube plates duringthe mounting of the bundle, which may be performed by mounting tubes insuccession from the central part towards the periphery or vice-versafrom the periphery towards the central part, it is possible to adopt aplan for piercing the various parts of the tube plates to allow thelevels of prestress and therefore the forces exerted by the tubes duringmounting to be increased. The tube plates are thus preloaded at a lowerlevel during the initial steps of mounting, and this affords a certainstiffening of the tube plates which become better able to withstand theprestresses and forces exerted by the mounted tubes in later steps ofthe tubing process.

[0064] Such distributions of the prestresses exerted in the tubes may beachieved by tailoring the diameters with which the holes passing throughthe tube plates are pierced. In particular, the diameters of the holespassing through the tube plates for mounting tubes at the beginning oftubing may be generally smaller than the diameters of the holes formounting tubes at the end of tubing.

[0065] In the case of circular tube plates, the piercing may be achievedin such a way that the holes passing through the tube plates at theircentral part have different diameters from the holes passing through thetube plates in their peripheral part.

[0066] The invention is not strictly restricted to the embodiments whichhave been described.

[0067] The mounting and diametral expansion of the tubes may beperformed using any type of tool suited to the dimensions of the tubesand to the characteristics of the steam generator bundle.

[0068] The invention can be applied to heat exchangers other thanonce-through heat exchangers, provided that the tubes have at least onestraight part which can be subjected to longitudinal prestressing.

1. Method for prestressing, in longitudinal tension, tubes (4) of a heatexchanger (1) comprising at least one straight part and fixed at theirends (4′a, 4′b) in holes (5) passing through at least one tube plate (3a, 3 b), in which the tensile prestress is obtained by hydraulicallydiametrally expanding at least one end part (4 a, 4 b) of each of thetubes (4) of the heat exchanger fixed at its two ends (4′a, 4′b) in theat least one tube plate (3 a, 3 b) in such a way as to inducedeformation through longitudinal contraction of the tube (4) bysubjecting the internal surface of the tube to a hydraulic pressure andin such a way that the prestress of the tube is tailored to a precisevalue, characterized in that the value of the prestress in each of thetubes (4) of the heat exchanger (1) is tailored by altering the initialradial clearance (6) between the external surface of the end part (4 a,4 b) of the tube (4) and the internal surface of the hole (5) passingthrough the tube plate (3 a, 3 b) in which hole the end part (4 a, 4 b)of the tube (4) is engaged before diametrally expanding the end part (4a, 4 b) and in that the tube (4) is hydraulically diametrally expandedalong the entire end part (4 a, 4 b) of the tube which extendssubstantially over the entire length of the hole (5) passing through thetube plate (3 a, 3 b).
 2. Method according to claim 1, characterized inthat hydraulic diametral expansion is performed on a first end part (4a) of the tube (4) which part is introduced into a through-hole (5) in afirst tube plate (3 a) to which one end (4′a) of the tube (4) is fixed,the second end (4′b) of the tube (4) being free inside a through-hole(5) of a second tube plate (3 b), and in that a second diametralexpansion is performed on the second part of the tube (4 b) which partis introduced into the through-hole (5) in the second tube plate (3 b),the tube (4) being fixed, at its two ends (4′a, 4′b) respectively, tothe first tube plate (3 a) and to the second tube plate (3 b),respectively, the deformation through longitudinal contraction of thetube and the prestress being obtained solely during the second operationof diametrally expanding the second end part (4 b) of the tube (4). 3.Method according to claim 1, characterized in that a first diametralexpansion is performed on a first end part (4 a) of the tube (4) whichpart is introduced into the opening (5) of a first tube plate (3 a), anda second diametral expansion operation is performed on a second part (4b) of the tube (4) inside the opening (5) of a second tube plate (3 b),the tube (4) being fixed at a first end (4′a) and at a second end (4′b),respectively, to the first tube plate (3 a) and to the second tube plate(3 b), the deformation through longitudinal contraction of the tube (4)and the prestress being obtained cumulatively by the first diametralexpansion operation and the second diametral expansion operation. 4.Method according to any one of claims 1, 2 and 3, characterized in thatthe heat exchanger (1) is a once-through heat exchanger comprising afirst tube plate (3 a) and a second tube plate (3 b) which plates arefixed in arrangements spaced apart in the longitudinal direction of thestraight tubes (4).
 5. Method according to any one of claims 1 to 4,characterized in that the initial radial clearance (6) between theexternal surface of the end part (4 a, 4 b) of the tube (4) and theinternal surface of the hole (5) passing through the tube plate (3 a, 3b) is altered by tailoring the diameter with which the hole (5) throughthe tube plate (3 a, 3 b) is pierced, bearing in mind the tolerances onthe outside diameter of the tube (4).
 6. Method according to any one ofclaims 1 to 5, characterized in that the at least one tube plate (3 a, 3b) has a number of through-holes (5) for the mounting of tubes (4) of aheat exchanger bundle and in that the prestress in the various tubes (4)of the heat exchanger bundle is tailored by altering the diameters withwhich the holes (5) through the tube plate (3 a, 3 b) are pierced. 7.Method according to claim 6, characterized in that the heat exchanger(1) is a once-through heat exchanger comprising two tube plates (3 a, 3b) of circular overall shape placed parallel to one another at a certaindistance apart and through which there pass holes (5) which areuniformly distributed between a central zone and a peripheral zone ofthe circular tube plate (3 a, 3 b), characterized in that the initialradial clearance (6) between the end parts of the tubes (4) and thesurface of the openings (5) is altered by piercing openings (5) withdiameters determined according to the clearance and the prestressrequired for the tubes fixed in the holes (5) distributed around thetube plate (3 a, 3 b).
 8. Method according to claim 7, characterized inthat the holes (5) intended to accommodate the tubes with which thebundle of the heat exchanger (1) begins to be assembled are generallypierced with a smaller diameter than the holes (5) intended toaccommodate the tubes (4) of the bundle of the heat exchanger (1) withwhich assembly of the bundle terminates.
 9. Method according to claim 8,characterized in that the holes passing through the tube plates (3 a, 3b) have diameters which are generally different in the central part andin the peripheral part of the circularly-shaped tube plate.
 10. Methodfor prestressing, in longitudinal tension, tubes (4) of small diameterand long length comprising at least one straight part, so as to increasethe buckling margin and reduce induced vibrations, and which are fixedat their ends (4′a, 4′b) in holes (5) passing through at least one tubeplate (3 a, 3 b), in which method the tensile prestress is obtained byhydraulically diametrally expanding the ends (4 a, 4 b) of each of thetubes (4) so as to induce deformation through longitudinal contractiondistributed uniformly along the length of the tube (4) by subjecting theinternal surface of the tube to a hydraulic pressure and in such a waythat the prestress of the tube is tailored to a precise value bypredetermined diametral expansion of the end part (4 a, 4 b) and thatthe tube (4) is hydraulically diametrally expanded.